8 ELEMENTS OF BIOLOGY 



of the atoms of the various elements are determined by the number 

 and arrangement of the electrons in the outermost shell or orbit, 

 and by the ease with which these outer electrons may be detached, 

 or by their tendency to take up more electrons. The arrangement is 

 conventionally represented by diagrams. Thus in the simplest case, 

 the hydrogen atom, the arrangement of the single proton and the 

 single electron may be: 



It is thought that in the oxygen atom the electrons form two 

 shells, the inner containing two, and the outer six electrons: 



It is known that the six electrons on the outer shell of the oxygen 

 atom do not entirely satisfy the electrical conditions in the system; 

 it will associate itself with two more electrons. Thus when oxygen 

 and hydrogen are mixed they combine promptly, the oxygen atom 

 attaching two hydrogen atoms by reason of the two electrons thus 

 obtained. The combination of atoms formed in this fashion is called 

 a MOLECULE. One speaks of the number of bonds by which an atom 

 attaches to other atoms as the valence of that particular substance. 

 Thus hydrogen has the valence i, oxygen 2, chlorine i, carbon 4, 

 and so on. The valence of an element, then, simply states the num- 

 ber of electrons that an atom will give or take. 



